Automotive design and manufacture present a unique set of problems in the selection of materials and processes used in the manufacture of automobile bodies. As a form of transportation, automobiles are unique because most buyers want a vehicle to have a certain individual styling. A recent trend in the automobile industry is toward production of distinctive vehicles styled to attract specific groups of consumers. This change has required the car builder to shift production from a few models manufactured in large volumes to a larger number of more distinctive body styles. These developments have demanded from the manufacturer both styling flexibility and reasonable tooling costs for each body style.
For many years, the primary method of applying a paint coat to an automobile was through a spray painting process. However, several recent developments make the spray painting process more difficult. First, many states have enacted environmental protection laws restricting the amount of volatile organic content (VOC) a paint may contain. Second, spray painting is more suited to larger production runs where unique body styling is not needed. Third, higher energy costs have driven up costs for production paint lines where high temperature ovens must be used to cure the paint coat. These factors are especially pronounced in two tone painting applications, where the automobile must pass through the paint line twice.
Development of a production process to replace the spray painting line requires addressing a number of technical problems. A potential replacement process must provide both environmental and cost advantages to the automaker before the process can receive consideration. It must also meet all of the quality standards for exterior automotive coatings. For example, a coating must have the appearance and durability characteristics comparable to a spray painted coating. It must also possess a number of important physical properties such as: high gloss, hardness, distinctness-of-image (DOI), abrasion resistance, weatherability (such as UV resistance), impact strength, thermal stability, gasoline and acid resistance, cleanability, adhesion to the substrate, resistance to water and humidity exposure, and hiding ability or opacity of the coating.
In the past, a conventional production process for applying an exterior paint coat to car bodies made of sheet metal has involved transporting the preformed auto bodies to application lines in the plant where the car bodies are dipped in paint, followed by transporting them to a separate location for baking the paint coat and waiting until the hardened paint coat dries thoroughly. Most paint systems today are acrylic enamels which are cross-linked into a hard, glossy, durable paint coat during baking. Following painting, the car bodies are transported back to the production plant for further assembly operations. Conventional painting of sheet metal car bodies is expensive and is a significant factory problem with respect to environmental protection, worker safety, corrosion treatment, and waste disposal. It has been estimated that about one-third of the total capital investment in an automobile production facility today is involved in painting car body members and panels.
If a coating process can be developed as an alternative to conventional auto painting, then automobile assembly plants can be more compact, and environmental and safety problems associated with conventional car painting at the factory site may be avoided.
Many corporate product development efforts have sought alternatives to conventional painting of exterior car body panels and parts. A number of problems must be overcome to develop an economical production-type paint system and process that can eliminate conventional spray painting operations. One approach, which is the subject of this invention, involves developing a paint-coated film which can be used to replace a conventional painted finish on the exterior of an automobile body. The film is made from a paint coat applied to a casting sheet. The paint coat is dried and then transferred from the casting sheet to the exterior car body panel. The coating operation permits use of high temperature-resistant paint systems capable of producing a tough, glossy finish. The film is coated with a pressure sensitive adhesive which allows the film to be transferred from the casting sheet and bonded to the exterior car body panel. One use of such a paint-coated film is to replace the separate painting step of a conventional two-tone paint coat process.
A number of technical problems must be overcome in order to use such a film for exterior automotive applications. For instance, the film must be defect-free before being applied. The film also must initially be repositionable. Therefore, the adhesive should have a relatively low initial tack value which increases over time and therefore becomes increasingly more difficult to remove. The film must avoid long-term delamination at the adhesive interface between the film and the car body panel; and inter-layer delamination between the various coatings in the finished composite film also must be avoided.
Durability properties are also critical in producing a paint coat capable of exterior automotive use. The paint coat must avoid exhibiting defects when exposed to mechanical impact and avoid deterioration of the surface from exposure to chemicals and to the weather. Among other properties, the film requires good cleanability, UV and heat resistance.
A paint system that produces the toughness or hardness necessary for exterior automotive use also must have the elongation properties necessary for applying the film around complex three-dimensional shapes without cracking, or producing stress lines or other surface non-uniformities. Certain paint films having good elongation at elevated temperatures are not necessarily applicable to the present invention which requires good elongation at room temperatures.
Thus, the desired paint system should have a critical combination of many physical properties in order to produce a surface capable of exterior automotive use, while retaining the desired surface characteristics after the application process. However, some physical properties tend to be mutually incompatible in such a process. For instance, a paint system may have good durability properties such as hardness, toughness, weatherability and the like; but the same paint system may not have sufficient room temperature elongation to be applied smoothly over a complex shape by a pressure-sensitive adhesive. Some paint systems have sufficient elongation to permit application over a complex shape, but they are too soft and therefore lacking in the necessary hardness and/or durability properties.
In summary, there is a need for an economical production process for decorating contoured exterior car body members or panels with a paint coat having both the durability and appearance properties necessary for exterior automotive use. Techniques for applying a paint coat to such a part can provide a valuable alternative to conventional spray painting of exterior car body members. Capital costs also can be reduced and environmental problems can be alleviated. Applying such a paint film requires, however, a paint system and processing techniques capable of producing a finished film having a combination of good durability, elongation, opacity, gloss and DOI levels, and a defect-free surface. The present invention solves these problems.